1. Field of the Invention
This invention relates to an adhesive for polarizing plate. The invention further relates to a polarizing plate and manufacturing method thereof using the adhesive for polarizing plate. The polarizing plate alone or an optical film obtained by laminating the polarizing plate can constitute an image display such as a liquid crystal display, an organic EL display or PDP.
2. Description of the Related Art
In the liquid crystal displays, for example, it is indispensable to dispose polarizers on both sides of a glass substrate providing a surface of a liquid crystal panel according to an image formation scheme adopted in the display. A polarizer is generally obtained in a procedure in which a polyvinyl alcohol-based film is dyed with a dichroic material such as iodine, thereafter, the film is crosslinked with a crosslinking agent and then, mono-axially stretched to thereby form a film. Since the polarizer is manufactured by stretching, it is easy to shrink. Since a polyvinyl alcohol-based film comprises a hydrophilic polymer, the film is very easily deformed especially in a humidified condition. Since the film itself is weak in mechanical strength, there has been a problem that the film is torn. Hence, adopted is a reinforced polarizing plate manufactured in a procedure in which a transparent protective film or transparent protective films each made from triacetyl cellulose or the like are adhered to on one side or both sides of a polarizer. The polarizing plate is manufactured by adhere the transparent protective film onto a polarizer using an adhesive.
In recent years, the range of uses of liquid crystal displays has been increased and extended to cover from portable terminals to large screen home TVs, and their standards have been defined for each application. In particular, portable terminal applications, which must be portable for users, strongly demand durability. For example, polarizing plates are required to have water resistance to such an extent that their properties and form will not change even under humidified conditions such as conditions causing dew condensation.
As mentioned above, a polarizer may be reinforced in strength with a transparent protective film to form a polarizing plate before use. As an adhesive for polarizing plate used in adhesion of the polarizer and the transparent protective film to each other, an aqueous type adhesive is conventionally preferable and, for example, a polyvinyl alcohol-based adhesive obtained by mixing a crosslinking agent into a polyvinyl alcohol aqueous solution has been employed. A polyvinyl alcohol-based adhesive may cause peeling at the interface between a polarizer and a transparent protective film in a humidified environment. This is considered because a polyvinyl alcohol-based resin, which is a main component of the adhesive, is a water-soluble polymer and a possibility of dissolution of an adhesive occurs in a situation of dewing. In order to cope with the problem, a proposal has been offered of an adhesive for polarizing plate containing a polyvinyl alcohol-based resin having an acetoacetyl group, and a crosslinking agent (see, for example, Japanese Unexamined Patent Publication (JP A) No. 7-198945).
The preparation of polarizing plates has a problem in which knicks (knick defects) can occur when a polarizer is bonded to a transparent protective film with the above-mentioned polyvinyl alcohol-based adhesive interposed therebetween. Knicks are defects of local irregularities formed at the interface between the polarizer and the transparent protective film. Against such knicks, there is proposed a method that includes using, as a polarizer, a polyvinyl alcohol film with a controlled water content and with its surface treated with a calendar roll under specific conditions, and laminating the film and a transparent protective film (see JP-A No. 10-166519. Knicks are particularly easy to occur when a polyvinyl alcohol-based resin having an acetoacetyl group is used for the polyvinyl alcohol-based adhesive.